Not oflly must it be possible to draw cables
into completed conduit and trunking systems, but neither
the cables nor their enclosures must be damaged in the process.
If too many cables are packed into the space available,
there will be a greater increase in temperature during operation
than if they were given more space. It is important to appreciate
that grouping factors (see
{4.3.5}) still apply to cables enclosed
in conduit or trunking.

To calculate the number of cables which
may be drawn into a conduit or trunkmg, we make use of four
tables ({Tables
4.14 to 4.17}). For situations not covered by these
tables, the requirement is that a space factor of 45% must
not be exceeded. This means that not more than 45% of the
space within the conduit or trunkmg must be occupied by
cables, and involves calculating the cross-sectional area
of each cable, including its insulation, for which the outside
diameter must be known. The cable factors for cables with
thermosetting insulation are higher than those for pvc insulation
when the cables are installed in trunking, but the two are
the same when drawn into conduit (see
{Table 4.14})

Fig 4.20 Low voltage
luminaire on lighting trunking

The figures in {Table
4.14} may
be high when applied to some types of plastic trunking due
to the large size of the internal lid fixing clips.

To use the ({Tables
4.14 to 4.17}). the cable factors for all the conductors
must be added. The conduit or trunking selected must have
a factor (otherwise called 'term') at least as large as
this number,

Example 4.9

The following single-core p.v.c. insulated cables
are to be run in a conduit 6 m long with a double set: 8
x 1,4 x 2.5 and 2 x 6 mm². Choose a suitable size.

Table 4.14 - Cable factors (terms)
for conduit and trunking

Type of conductor

Conductor c.s.a. (mm²)

Factor for conduit

Factor for trunking pvc insulation

Factor for trunking thermosetting
insulation

Solid

1.0

16

3.6

3.8

Solid

1.5

22

8.0

8.6

Stranded

1.5

22

8.6

9.1

Solid

2.5

30

11.9

11.9

Stranded

2.5

30

12.6

13.9

Stranded

4.0

43

16.6

18.1

Stranded

6.0

58

21.2

22.9

Stranded

10.0

105

35.3

36.3

Stranded

16.0

145

47.8

50.3

Stranded

25.0

217

73.9

75.4

Table 4.15 - Cable factors (terms)
for straight runs up to 3m.

Type of conductor

Conductor c.s.a. (mm²)

Cable factor

Solid

1.0

22

Solid

1.5

27

Solid

2.5

39

Stranded

1.5

31

Stranded

2.5

43

Stranded

4.0

58

Stranded

6.0

88

Stranded

10.0

146

Table 4.16 - Conduit factors
(terms)

Length of run between boxes (m)

1

2

3

4

5

6

8

10

Conduit, straight

-

16mm

290

290

290

171

171

167

158

150

20mm

460

460

460

286

278

270

256

244

25mm

800

800

800

514

500

487

463

442

32mm

1400

1400

1400

900

878

857

818

783

Conduit, one bend

-

16mm

188

177

167

158

150

143

130

120

20mm

303

286

270

256

244

233

213

196

25mm

543

514

487

463

442

422

388

258

32mm

947

900

857

818

783

750

692

643

Conduit, two bends

-

16mm

177

158

143

130

120

111

97

86

20mm

286

256

233

213

196

182

159

141

25mm

514

463

422

388

358

333

292

260

32mm

900

818

750

692

643

600

529

474

F

or 38mm conduit
use the 32mm factor x 1.4.
For 50mm conduit use the 32mm factor x 2.6.
For 63mm conduit use the 32mm factor x 4.2.

{Table
4.16} gives a conduit factor for 20 mm
conduit 6 m long with a double set as 233, which is less
than 364 and thus too small. The next size has a conduit
factor of 422 which will be acceptable since it is larger
than 364.

The correct conduit size is 25 mm diameter.

Example 4.10

The first conduit from a distribution board will be
straight and 10 m long. It is to enclose 4 x 10 mm² and
8 x 4 mm² cables. Calculate a suitable size.

From {Table
4.14}, cable factors are 105 and 43 respectively.
Total cable factor:

=(4 x 105) + (8 x 43) = 420 + 344 = 764

From ({Table
4.15}, a 10 m long straight 25 mm conduit
has a factor of 442. This is too small, so the next size,
with a factor of 783 must be used.